In the aeronautics field, an important normative system exists due to the criticality of a potential accident. Each aircraft is forced to meet a set of rules in order to be certified and thus allowed to fly and convey passengers. This regulation can change according to the problems encountered in flight and also the materials used and the manufacturing methods. The justification methods have also changed even if a major part is still performed by tests. The changes have always been performed in order not to deteriorate the passenger safety level.
The certification of an aircraft indicates that the latter meets the navigability regulatory requirements. Its structure must withstand the exceptional loads (static strength) that are encountered, but also the application of repeated loads (endurance). For the requirements, the demonstration of a safe behavior up to the limits of the specified field is required. Other parts of the regulation include requirements related to the bird strike resistance. Thus, the manufacturers have to demonstrate to the certification authorities that each sensitive component of an apparatus, such as the cockpit, the foredeck and the wing leading edge, which may be struck by a bird, withstands such strike or at least such strike does not affect in a catastrophic manner the flight safety and the landing of the aircraft. The authorities also require that the final test be performed with an actual bird. For the sake of reproducibility and uniformity, and for avoiding the use of live or dead birds, artificial birds, also called substitute impactors, have been developed and are used as a substitute for actual birds. The idea of a substitute impactor has been studied since about twenty years but, although the manufacturers are now used to employ it during the phases of designing the parts, none of them have manifested the will to use it for the certification tests. The emergence of dynamic calculation over the past years should give a second life to the substitute impactor, which can be modeled far more easily, and encourage the manufacturers to use it for the development and certification tests, to the extent that it could be validated by the entitled authorities of European and American civil aviation (particularly EASA and FAA).
Some of these impactors are constituted by a mixture of water and gelatin and are in particular used for simulating the suction of birds by a reactor. However, this gelatin-based mixture has to be thick enough to allow to be handled and projected on the target such as a reactor or an aircraft wing. This can be provided by mixing a significant amount of gelatin with hot water, and then letting the mixture stand such that the resulting gel is sufficiently rigid, solid and stable for allowing it to be fired at a speed representative of specific conditions of actual strikes of birds on flying aircrafts.
However, a disadvantage is that the gel tends to have a rubbery consistence, which does not entirely reproduce the actual behavior of a bird. The high level of elasticity can cause the rebound or an unusual crushing of the impactor upon striking while an actual bird would not do it.
Furthermore, the projectiles are fired into the areas of the elements to be tested at a high speed, for example using a gas gun, such as using compressed air. Due to the high speeds and the high air resistance resulting therefrom during the projectile flight phase, the impactor is deformed, which affects the quality of the simulation of an actual bird and the quality of the tests, if only due to the changes made to the trajectory and/or to the speed of the projectile, in particular regarding the measurement of the latter that needs to be acquired.
To prevent these disadvantages, U.S. Pat. No. 8,220,396 describes a projectile comprising a stabilizing structure, for example made of cardboard and with a honeycomb shape, surrounded by gel formed, for example, from water and gelatin or a compound similar to a jelly, such as rubber, silicone, soap glycerin, starch, polymer gel, rubber, latex and/or modelling paste.
This stabilizing structure, on one hand, has a high rigidity for preventing the projectile from deforming during flight and, on the other hand, is very fragile such that it is almost immediately destroyed upon striking the target and thus, according to the author, has almost no influence on the behavior of the projectile upon striking.
Patent application US2010/0077832 describes a projectile comprising a solid foam and a gel. The foam phase is generally a polymer foam such as for example phenolic resin foam, polyurethane foam, polyester sponge foam or urea-formaldehyde resin foam.
This foam can have an open structure to allow the gel, when it separates, to be suctioned, by capillarity, into the pores of the foam, the porosity of the latter being preferably higher than 80%. It is indicated that this foam can simulate the skeleton of a bird.
The gel is preferably an aqueous gel. The gelling agent can be selected from the following list: gelatin, agar, carrageenan, pectin, konnyaku, carob gum, alginates, gellan gum, hypromellose, hydroxypropylmethyl cellulose, xantham gum and starch. Preferably, the gelling agent is gelatin. It can be absorbed entirely or not within the foam.
The projectiles according to these patents have a good capacity to retain their sizes in flight, but add additional elements which make the projectile manufacture more complex and, regardless of the authors, add elements which affect the quality of the tests and especially their similarity to a live bird.